New American automobiles have incorporated fuel vapor storage canisters for some time now, originally designed simply to capture fuel vapor that would otherwise have vented from the tank while it was capped. Lately, the same canisters have also been used to capture fuel vapor displaced while the tank is uncapped and being filled with liquid fuel. Regardless of source, the stored fuel vapors are purged from the canister while the engine is running, typically by a vacuum (negative pressure air flow) applied to the canister, which pulls outside air in and through the activated carbon bed of the canister to desorb previously stored fuel vapors. The desorbed vapor is ultimately pulled into the engine air induction system and burned. The process is reversed when vapors enter the canister from the fuel tank, with clean air being expelled to the outside atmosphere. Older canister designs generally incorporated a single, common port to atmosphere that provided both the outside air inlet for purge and the clean air exit for fuel adsorption. Such older designs also typically had the single atmospheric inlet/exit port located at one end of the canister, with the fuel vapor inlet and purge ports located at the opposite end. Newer canister designs often divide the activated carbon bed roughly in half with a flow division baffle, and locate the purge and fuel vapor ports on one side (vapor side) of one canister end, and the common atmospheric air inlet/exit port on the other side (air side) of the same end.
While most disclosed canister designs show the air inlet/exit port as opening directly to atmosphere, it has also been suggested for some time that the common port run through a filter. Such a filter would be useful for cleaning the atmospheric air that was pulled into the canister during purge, but of no real use for filtering the air vented to the outside during fuel adsorption, when it would merely serve as an air flow impediment. It has also been known for sometime to use the engine air cleaner housing as a convenient filter for the inlet air, since it is a filter that already exists. However, with a common inlet/exit port, fuel vapors could be inadvertently fed into the air cleaner during vapor fill of the canister, if the carbon bed's capacity were exceeded and vapors "broke through". This could lead to an over rich fuel air mixture being fed to the engine at start up.
To solve this problem, it has been proposed to divide the single air inlet/air exit port into two separate valved ports, each spring biased closed in opposite directions. An inlet port allows outside air to be pulled only through the air cleaner during purge, and the exit port allows air (and any excess break through fuel vapor) to vent directly to atmosphere, rather than to the air cleaner. An example may be seen in U.S. Pat. No. 5,590,634, in FIG. 2 thereof. A so called "atmospheric valve 18" incorporates a single valve housing at the top of the air side of the canister. An air inlet port 18a is closed by a diaphragm 18b loaded thereagainst by a compression spring 18c. An air inlet line 18e from air cleaner 3 connects to the air inlet port 18a across the diaphragm 18b. An oppositely spring loaded check valve 19a in the same housing opens directly to atmosphere. The spring 18c loading the diaphragm 18b down to block the air path to the air cleaner 3 must be stronger than the spring loaded check valve 19a, in order to allow air any break through vapor to pass only through valve 19a to atmosphere, and not to air cleaner 3. Consequently, during purge, in order to successfully pull the diaphragm 18b open, a separate pipe 18g must be provided to route some of the negative pressure from the purge port back behind the diaphragm 18b to overcome the spring 18c and allow air inlet flow to begin from the air cleaner 3. So, the strength of the spring 18c represents a compromise between certain purge air entry from the air cleaner and certain exit of venting air to the atmosphere only, without reverse flow back to the air cleaner.
An even more recent development in fuel vapor control systems has been the requirement for so called "on board" or self-contained, automatic diagnostic programs to periodically assure that the systems are working as intended. Testing the system against external air leaks is an important part of that assurance. Generally, this involves sealing off any check valve type air inlets to the fuel tank, or canister, or both at once, with solenoid operated blocking valves. Next, purge vacuum is applied to the canister create a known level of negative pressure, and then measuring the pressure again at a set time interval later to see whether it has been sufficiently retained. This indicates no serious air leaks. In the system disclosed in U.S. Pat. No. 5,590,634 noted above, the diaphragm 18b alone is relied upon to block the air inlet to the canister during diagnostic evacuation. As a consequence, the level of negative pressure that can be applied and retained by the canister is effectively limited by the setting pressure of the diaphragm valve 18b, that is, by the strength of the spring 18c. A negative pressure greater than the strength of spring 18c, since it is deliberately routed around and behind the diaphragm 18b to open it during the normal purge operation, will also overcome spring 18c and open it during diagnostic evacuation, and thereby pull outside air back into the canister. This leak will continue until the negative pressure falls low enough to allow the spring 18c to push the diaphragm 18b closed again.